This invention relates to traveling water screens and particularly to a non-metallic basket frame for a traveling water screen.
Traveling water screens have been used for many years in applications in which it is necessary to screen debris and fish from large volumetric flows of water. A typical application is a power plant which requires a continuous, large volume stream of cooling water. Normally, the water for this purpose is taken from the ocean or from rivers or lakes through an inlet water channel. Debris and fish also enter the channel with the water and must be screened out of the water stream to prevent debris from clogging the condenser tubes and prevent the fish from being killed in large numbers by heat and impingement on the condenser tubes and other parts of the cooling system. The screening operation must present the minimum possible obstruction to the water flow and must preserve, to the extent possible, the life and viability of the fish.
In the past, a typical traveling water screen used coarse mesh wire (9.5 mm). In recent years, the utilities have begun to recognize that a finer wire mesh in the water screen reduces the problems of blockage and constriction of the water flow path by growth of marine organisms that the coarse screen failed to remove. Accordingly, consideration has been directed to a more efficient screen which incorporates fine mesh wire (1.0 mm). However, the fine wire mesh removes far more material from the water than the coarse wire, so the screening mechanism must be operated more frequently or continuously to avoid "blinding" or complete blockage of the screen by debris which would produce a serious headloss condition. The finer the wire is, the greater the cleaning requirements are and the greater the headloss produced by the wire itself.
A traveling water screen provides the means for removing debris and safely removing fish from the incoming water, for preserving the life of the fish, and for cleaning the screen. It includes a wide belt-form structure made up of a large number of elongated rectangular baskets attached at their ends to a pair of chains which are trained around a pair of head sprockets at the top and a pair of traction wheels at the bottom. The sprockets are driven so that the baskets continuously or periodically lift out of the water on the upstream side taking the impinged fish and debris with them. Fish are then gently removed by a low pressure water spray, and the debris is removed by a high pressure water spray. The fish and debris are deposited in separate troughs and the fish are returned to the water source. The baskets descend on the downstream side, pass around the traction wheel and ascend again on the upstream side. In order for the traveling water screen to effectively remove debris and fish from the water stream, it is necessary that the joints between adjacent edges of the baskets, and the joints between the baskets and the frame be kept small to prevent ingress of debris and fish.
Numerous problems have been identified and have existed for many years with respect to the operation and maintenance of traveling water screens. One persistent problem is the loosening of the fasteners that attached the basket to the support chains. The metal structures in a traveling water screen are very heavy and the vibration produced in its operation above the water level is almost entirely undamped. As a consequence, there has been a persistent complaint among operators of traveling water screens that the baskets become loose or actually fall off, and frequent retorquing of the attachment bolts is necessary to prevent this from occurring.
One of the most serious problems of prior art traveling water screen baskets is corrosion. Basket frames are most commonly fabricated of a welded carbon steel construction which produces the most inexpensive basket. However, the carbon steel frame must be carefully cleaned and painted to prevent rapid corrosion and weakening of the basket frame. Even with careful surface preparation and protective coating, and most common cause for basket replacement is corrosion because the protective surface coating becomes scratched and abraded under the severe conditions that it encounters in use. To resolve this problem, some installations have elected to use basket frames formed of stainless steel because of its corrosion resistance. However, stainless steel baskets are extremely expensive, and are subject to corrosion, albeit at a slower rate than carbon steel.
The necessity for periodic maintenance and inspection of the baskets of a traveling water screen is the source of another area in which traveling water screens have long needed improvement. The conventional traveling water screen basket typically weighs about 275 pounds; it is a heavy and awkward burden to carry and attach to the traveling water screen support chains. Four workers are normally required to remove or replace a water screen basket, and the process is fatiguing because of the great weight involved, and the awkward and cramped working space within the intake area of a traveling water screen. The process is difficult and tiring, and therefore the work goes slowly.
A traveling water screen can typically extend 40 feet between the head shaft and the foot shaft, in which case each traveling water screen would require 46 baskets. Removal and replacement of 46 baskets typically takes about two working days, during which time the water screen is out of operation. This increases the burden on the other water screens in the installation and increases the chance that they will become jammed or blocked and also increases the headloss across the other water screens.
A high headloss across the water screen can produce severe mechanical stress on the water screen structures. If the stresses become high enough or the structures have been weakened by corrosion, a failure can occur somewhere in the system which can cause the water screen to jam in one fixed position. This often requires the use of divers to locate and burn out the failed structure and then either replace it under water or dismantle it and take it to the surface for repair. This can be a very lengthy and expensive procedure and is the primary reason why periodic maintenance on the traveling water screen is performed at frequent intervals.
To prevent such failure caused by corrosion weakening of the basket, it would be desirable to form the basket of non-metallic, corrosion resistant material, but this has never been successfully accomplished even though it has been attempted occasionally. One of the most serious reasons for concern about non-metallic baskets is the requirement that the basket lip be extremely rigid and resistant to deflection under the forces exerted by a high headloss. If a high headloss condition exists and the basket deflects under that force, it is possible for debris and fish to pass between the adjacent lips of adjacent baskets which defeats the purpose of the water screen. This deflection can also cause the lip and possibly the end plates to crack or break because of the twisting action at the end of the lips where they attach to the end plates. High deflection can cause the basket frame to interfere with the cross members of the water screen frame. This could cause the screen to jam and possibly necessitate major repairs.
The great weight of the chain and the basket frames in a conventional traveling water screen requires that the head shaft sprocket, the head shaft bearings and associated structures be robust and conservatively engineered to carry the weight and still provide a satisfactory service life at a reasonable cost. A lighter weight basket would make it possible to increase the service life at no increased cost, or to decrease the size of some component parts to provide an equal or better service life at less cost. Reductions in dead weight will also decrease maintenance and down time for major overhaul and repairs.
Since weight of the chain and basket train has such a strong influence on the design and cost of the support apparatus and maintenance, it would be desirable to reduce the weight of the basket train even further than would be accomplished by use of non-metallic materials. In addition, the time required by divers to remove the baskets when that procedure is necessary would be greatly shortened and their fatigue cycle greatly lengthened if it were possible to make the weight of the baskets in water neutral or even buoyant so that they could be easily handled under water by divers.